The present invention relates to the machining of cylinder crankcases of an internal combustion engine.
More specifically, the invention relates to a method for machining a connecting rod passage in a cylinder crankcase of an internal combustion engine, the crankcase and the engine produced in accordance with this method.
The method involves producing at the base of the cylinder shaft a straight cut of cylindrical form, which is capable of allowing the lateral travel of the connecting rod when the piston moves up and down in the cylinder, with a rotary milling tool which operates by means of travel below the cylinder.
A connecting rod of an internal combustion engine generally comprises a connecting rod little end, which carries the drive shaft of the piston, and a connecting rod big end which is connected to the crankshaft. The connecting rod little end has a bore which receives the drive shaft of the piston. The connecting rod big end also has a bore which receives a crankpin which is offset relative to the rotation axis of the crankshaft.
The connecting rod serves to connect the crankshaft to the piston in order to convert the rotation movement of the crankshaft into an alternating vertical displacement of the piston inside the cylinder. The big end thereof rotates with the crankshaft, whilst the little end thereof is displaced vertically with the piston between the positions of top dead centre and bottom dead centre, shown in FIG. 1. Between these two extreme positions, the connecting rod is raised and lowered with the piston, moving towards and moving away from the wall of the cylinder.
The openings which allow the travel of the connecting rod at the cylinder base, or connecting rod passages, are generally machined at the base of the cylinder shaft, using a rotating milling cutter which moves by means of vertical downward movements. The milling cutter acts on the base of the shaft from the front and the cut, or recess, formed in this manner is a straight cut in the lower portion thereof.
The frontal action on the cylinder shaft by the milling cutter involves a combined risk of rapid wear of the milling tool and of the skirt of the piston when it moves downward into contact with the recess.